Golf balls have undergone extensive evolution designed to improve their play-related characteristics, such as durability, distance, and control. Modern golf balls can be classified as one-piece, two-piece, and three-piece balls.
One-piece balls are formed from a homogeneous mass of material with a dimple pattern molded therein. One-piece balls are inexpensive and durable, but do not provide great distance because of relatively high spin and low velocity.
Two-piece balls are the most popular type of ball in use today. They comprise a cover molded around a solid core. Conventionally, both two-piece and three-piece golf balls are made by molding a cover about the ball core, either by injection-molding cover stock material around a core held in a retractable pin mold or by compression molding preformed half-shells about the core. The preformed half-shells are formed by injecting cover stock material into half-shell molds and solidifying the cover stock material into a corresponding shape. Golf ball cores, whether wound or solid, typically measure from 1.4 to 1.6 inches (3.5 to 4.1 cm) in diameter. The cover is molded about the core to form a golf ball having the minimum United States Golf Association (USGA) specified diameter of 1.68 inches (4.3 cm). Typically, the cover has a thickness of about 0.060 inches. Two-piece balls typically have a hard "cutproof" cover which gives a longer distance ball, but which has lower spin rates, resulting in a decreased ability to control the ball.
Three-piece or wound balls are made by molding a cover about a wound center. The center is typically made of rubber and can be solid, semi-solid or fluid, e.g., liquid-filled. A wound center is prepared by winding a thin thread of elastic material about the center. The wound center is then covered with a durable cover material. Wound balls are generally softer and provide more spin, resulting in increased control over the ball, but such balls typically travel a shorter distance than that traveled by a two-piece ball. As a result of their more complex construction, wound balls generally are more expensive to produce than two-piece balls.
The covers of golf balls are generally made from a variety of materials, such as balata or ionomer resins such as SURLYN.RTM. and IOTEK.RTM.. Balata, which is a natural or synthetic trans-polyisoprene rubber, is the softest of these cover materials. Balata covered balls are favored by the more highly skilled golfers because the softness of the cover allows the player to achieve spin rates sufficient to more precisely control ball direction and distance, particularly on shorter shots.
However, balata covered balls are expensive and less durable as compared to the other covering materials. In particular, balata covered balls are subject to nicks or cuts as a result of a mis-swung golf club or due to landing on cart paths, etc. and/or contact with rocks, trees, etc. Such nicks or cuts detract from the flight characteristics of such balls, rendering them of little use. Accordingly, cover compositions have been developed in an attempt to provide balls with spin rates and a feel approaching those of balata covered balls, while also providing a golf ball with a higher durability and overall distance.
Ionomer resins have, to a large extent, replaced balata as a cover stock material. Chemically, ionomer resins are a copolymer of an olefin and an alpha, beta ethylenically-unsaturated carboxylic acid having 10-90% of the carboxylic acid groups neutralized by a metal ion. See U.S. Pat. No. 3,264,272. Commercially available ionomer resins include, for example, copolymers of ethylene and methacrylic or acrylic acid. These are sold by E.I. DuPont de Nemours and Co. under the trademark "SURLYN.RTM." and by the Exxon Corporation under the trademark "ESCOR.RTM." and the trademark "IOTEK.RTM.". These ionomer resins are distinguished by the type of metal ion, the amount of acid, and the degree of neutralization. Also, Chevron Chemical Co. sells a family of ionomers produced from ethylene acrylate-based copolymers under the trademark "IMAC.RTM.".
U.S. Pat. Nos. 3,454,280, 3,819,768, 4,323,247, 4,526,375, 4,884,814, and 4,911,451 all relate to the use of SURLYN.RTM.-type compositions in golf ball covers. However, while SURLYN.RTM. covered golf balls as described in the preceding patents possess virtually cutproof covers, they have inferior spin and feel properties as compared to balata covered balls.
In 1986, DuPont introduced two new classes of ionomer resins. One was a sodium and zinc ionomer resin having a low flexural modulus. DuPont suggested using and blending the same with other ionomer resins for making a golf ball cover. Golf ball covers made from these low flexural modulus ionomer resins have improved spin and feel characteristics but relatively low velocity. The other was a lithium ionomer resin which was a copolymer of ethylene and methacrylic acid. These lithium ionomer resins have a very high flexural modulus, typically about 60,000 psi (415 MPa). DuPont suggested that lithium ionomer resins could be used to produce a golf ball cover which would be more cut resistant and harder than a cover made with either sodium or zinc ionomer resins. DuPont also suggested that a golf ball having a cover made from a lithium ionomer resin would go farther, have a higher coefficient of restitution and be less prone to cutting (i.e., more durable) than a golf ball made from other known ionomer resins such as sodium and zinc ionomer resins and blends thereof. DuPont further suggested that lithium ionomer resins could be used in blends with other ionomer resins where they can impart better cut resistance to those other resins.
The USGA has promulgated a rule that no golf ball shall have an initial velocity that exceeds 255 feet (78 m) per second, i.e., 250 feet (76 m) per second with a 2% tolerance when impacted by the USGA test machine under specified conditions. Golf balls with covers made from ionomer resins with a low flexural modulus are woefully below this maximum and, as should be appreciated, all golf ball manufacturers strive to come as close as possible to this limit.
In various attempts to produce an ideal golf ball, the golfing industry has blended hard ionomer resins (i.e., those ionomer resins having a hardness of about 60 to about 70 on the Shore D scale as measured in accordance with ASTM method D-2240) with a number of softer polymeric materials, such as softer polyurethanes (see, e.g., U.S. Pat. No. 4,674,751 to Molitor et al.). However, the blends of the hard ionomer resins with the softer polymeric materials have generally been unsatisfactory in that these balls exhibit numerous processing problems. In addition, the balls produced by such a combination are usually short on distance.
In addition, various hard-soft ionomer blends, that is, mixtures of ionomer resins which are significantly different in hardness and/or flexural modulus, have been attempted. U.S. Pat. No. 4,884,814 discloses the blending of various hard methacrylic based ionomer resins with similar or larger quantities of one or more "soft" ionomer methacrylic acid based ionomer resins (i.e., those ionomer resins having a hardness from about 25 to 40 as measured on the Shore D scale) to produce relatively low modulus golf ball cover compositions that are not only softer than the prior art hard ionomer covers but also exhibit a sufficient degree of durability for repetitive play. These relatively low modulus cover compositions were generally comprised of from about 25 to 70% of hard ionomer resins and from about 30 to 75% of soft ionomer resins.
U.S. Pat. No. 5,324,783 discloses golf ball cover compositions comprising a blend of a relatively large amount, e.g., 70-90 wt. %, of hard ionomer resins with a relatively low amount, e.g., 10 to about 25-30 wt. %, of soft ionomers. The hard ionomers are sodium or zinc salts of a copolymer of an olefin having from 2 to 8 carbon atoms and an unsaturated monocarboxylic acid having from 3 to 8 carbon atoms. The soft ionomer is a sodium or a zinc salt of a terpolymer of an olefin having from 2 to 8 carbon atoms, methacrylic acid and an unsaturated monomer of the acrylate ester class having from 1 to 21 carbon atoms.
In order to approximate the characteristics of balata covered balls at lower cost, the art has developed balls having a variety of cover compositions. There are more than fifty commercial grades of ionomers available from DuPont and Exxon with a wide range of properties which vary according to the type and amount of metal cations, molecular weight, composition of the base resin (i.e., relative content of ethylene and methacrylic and/or acrylic acid groups) and additive ingredients such as reinforcements, etc. As noted above, these prior art compositions have a considerably higher cut resistance and durability as compared to balata covered balls. A great deal of research continues in order to develop golf ball cover compositions exhibiting not only improved impact resistance and carrying distance properties produced by the "hard" ionomeric resins, but also the playability (i.e. "spin") characteristics previously associated with the "soft" balata covers, properties which are still desired by the more skilled golfer.
Finishing coats of paints are often applied to the molded surface of the golf ball in order to obtain a desired whiteness for the ball and to protect logos and/or trademarks appearing on the surface of the golf ball cover materials. The abrasion and cut resistance of such coatings is typically adjusted by adding filler materials, by using a harder resin, that is, one having a higher glass transition temperature (Tg), by employing a slip and mar agent, and/or by increasing the crosslink density of the polymer.
U.S. Pat. No. 3,992,014 teaches that solid particulate filler materials, for instance 1-50% based on weight of rubber polymer, can be included in the formulation as needed or desired to impart specific properties to the molded article. For example, for golf ball applications, such filler materials as zinc oxide, magnesium oxide, silica, hydrated silica such as HiSil 233, carbon black, lithium oxide, and the like, can advantageously be used to improve the scratch and abrasion resistance of the composition. The '014 patent also teaches the inclusion of a silane in a homogeneous golf ball composition which is based on a cross-linked rubber polymer for the purpose of improving the velocity of such golf balls.
U.S. Pat. No. 4,486,319 teaches that the lower the Melt Flow Index of the coating polymer, the higher the abrasion resistance of the composition. The '319 patent also teaches that ionomers do have abrasion resistance, which makes them useful for such items as golf ball covers.
U.S. Pat. No. 5,197,740 teaches that Surlyn containing covers are highly advantageous in that the resulting covers are extremely resistant to cuts and abrasion. These balls are commonly referred to in golfing circles as cutless balls. However, the '740 patent also enumerates problems with the Surlyn balls: while extremely advantageous from the standpoint of cut resistance, the golf balls described in U.S. Pat. No. 3,454,280 have a shortcoming in that they do not have cold temperature cracking properties such as to permit their use at temperatures below freezing.
In fact, when such balls are played at temperatures below freezing, they have a tendency to crack or explode when struck by a golf club. Likewise, balls produced according to the patent are lacking in their coefficient of restitution, a property related to the distance which a golf ball can be driven.
U.S. Pat. No. 5,409,233 teaches that coatings which are prepared using hexamethylene diisocyanate-based products show resistance to chemicals and abrasion. The '233 patent also teaches the use of mar and slip agents.
None of the prior art patents teach a solution to the continuing problem of providing a golf ball with an industry acceptable coating suitable for use under rigorous conditions. In view of the problem caused by the cutting, scratching, and abrasion of the covers of golf balls, there exists a need for golf ball covers and coatings that are capable of providing improved cut and abrasion resistance and adherence without adversely affecting overall golf ball performance characteristics.
Relatively recently, a number of golf ball manufacturers have introduced multilayer golf balls, i.e., balls with multiple core layers, multiple intermediate or mantle layers and/or multiple cover layers, in an effort to overcome some of the undesirable aspects of conventional two-piece balls such as their hard feel, while maintaining the positive attributes of these balls such as their increased initial velocity and distance. The physical characteristics of the cover layer and the various intermediate layers will vary depending upon the construction as well as the selection of the materials for such layers.
Multilayer golf balls can be formed using a variety of constructions. For example, multilayer balls may have two or more cover layers molded around a conventional core. Alternatively, they may comprise a conventional cover and a core with one or more intermediate layers interposed between the cover and the core. Likewise, multilayer balls may be formed from cores having more than one core layer and may optionally contain one or more intermediate and/or cover layers. Multilayer balls may even comprise a conventional wound core around which at least one intermediate layer and/or at least one cover layer is formed. Typically, one or more of the layers is softer in relation to the other layers. Examples of multilayer balls include the Episode (Titleist), Altus Newing (Bridgestone), Giga (Spalding), Metal Mix (Dunlop) and Ultra Tour Balata (Wilson).
Golf ball cores are commonly formed from base elastomers such as polybutadiene, natural rubber, styrene butadiene rubber, and polyisoprene. Typically, golf ball cores are predominantly made of polybutadiene. In order to obtain the desired physical properties for golf balls, manufacturers have added cross-linking agents, such as metallic salts of an unsaturated carboxylic acid to the polybutadiene. The amount of cross-linking agent added is typically about 20 to 50 parts per hundred parts of polybutadiene. Most commonly, zinc diacrylate or zinc dimethacrylate are used for this purpose of these two cross-linkers, zinc diacrylate has been found to produce golf balls with greater initial velocity than zinc dimethacrylate.
As used herein, "pph" refers to parts per hundred parts of the base elastomer. Typically, about 5 to 50 pph (parts per hundred) of zinc oxide (ZnO) is also added to the composition. This material serves as both a filler and an activation agent for the zinc diacrylate/peroxide cure system. The zinc diacrylate/peroxide cure system, which is well known to those of ordinary skill in this art, cross- links the polybutadiene during the core molding process. The high specific gravity of zinc oxide (5.57) can serve the dual purposes of adjusting the weight of the golf ball, in addition to acting as an activation agent.